Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and personal digital assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charge Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid-state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect their delicate components and to provide external electrical contacts.
An image sensor generally includes an array of pixels arranged in a focal plane. Each pixel is a light sensitive element that includes a photogate, a photoconductor, or a photodiode with a doped region for accumulating a photo-generated charge. Microlenses and color filter arrays are commonly placed over imager pixels. The microlenses focus light onto the initial charge accumulation region of each pixel. The photons of light can also pass through a color filter array (CFA) after passing through the microlenses and before impinging upon the charge accumulation region. Conventional technology uses a single microlens with a polymer coating, which is patterned into squares or circles over corresponding pixels. The microlens is heated during manufacturing to shape and cure the microlens. Use of microlenses significantly improves the photosensitivity of the imaging device by collecting light from a large light-collecting area and focusing the light onto a small photosensitive area of the sensor.
Manufacturing image sensors typically includes “post-processing” steps that occur after the microlens array is formed on a workpiece. Accordingly, it is necessary to protect the microlens array during these post-processing steps to prevent the microlens array from becoming contaminated with particles that might be released during these steps. One approach to addressing the foregoing manufacturing challenge is to attach individual image sensor dies to a substrate, tape over the corresponding sensor arrays, and then use a molding process to form “standoffs” to which a cover glass is mounted. The cover glass can accordingly protect the image sensor during subsequent processing steps, and becomes part of the sensor package.
One drawback with this approach is that it is performed at the die level and accordingly cannot protect the sensor arrays during processing steps that occur before the dies have been singulated from a corresponding wafer or other larger workpiece. Another drawback with this approach is that a mold release agent is typically used to release the die from the mold machine in which the standoffs are formed. However, the mold release agent tends to inhibit the adhesion of adhesive compounds, which are required to attach the cover glass. Accordingly, the standoff surfaces must typically be cleaned (e.g., with a plasma process) before attaching the cover glass. This additional cleaning step increases the cost of manufacturing the die, and reduces manufacturing throughput.